The present invention relates to the use of a biaxially oriented polypropylene film as in-mold label in deep drawing.
Label films comprise an extensive and technically complex field. It is differentiated between different labeling techniques, which are entirely different with regard to process conditions and inevitably make different technical demands on label materials. All labeling processes have in common that as final result, optically pleasing labeled containers, in which good adhesion to the labeled container has to be ensured, have to result.
In the labeling processes, very different techniques are used to apply the label. It is differentiated between self-adhesive labels, wrap-around labels, shrink labels, in-mold labels, patch labeling, etc. The use of a film made of thermoplastic material as label is possible in all of these different labeling processes.
In in-mold labeling, it is also differentiated between different techniques, in which different process conditions are used. All in-mold labeling processes have in common that the label takes part in the actual molding process of the container and is applied during the process. However, very different molding processes are used here, such as for example injection molding, blow molding, deep drawing.
In the injection molding process, a label is inserted into the injection mold and back-injected with a plastic melt. By means of the high temperatures and pressures the label bonds to the injection-molded part and becomes an integral, non-detachable component of the injection-molded article. For example tub and lid of ice cream or margarine tubs are produced by this method.
Here, individual labels are taken from a stack or cut from a roll and inserted into the injection mold. The mold is designed in such a way that the melt stream is injected behind the label and the front of the film rests against the wall of the injection mold. During injection molding, the hot melt bonds to the label. After injection molding, the injection mold tool opens, the injection-molded article with label is ejected and cools. As a result, the label has to adhere to the container in wrinkle-free and visually flawless fashion.
During injection, the injection pressure is in a range of 300 to 600 bar. The plastic materials used have a melt flow index of around 40 g/10 min. The injection temperatures depend on the plastic material used. In some cases, the mold is additionally cooled to avoid that the injection-molded article sticks to the mold.
In blow molding of containers or hollow bodies, direct in-mold labeling is also possible. In this method, a molten tube is extruded vertically downwards through a ring-shaped die. A vertically split molding tool closes and encloses the tube, which is thereby squeezed shut at the bottom end. At the top end, a blow mandrel is inserted, through which the opening of the molded piece is formed. Via the blow mandrel, air is supplied to the warm molten tube so that it expands and conforms to the inner walls of the molding tool. In the process, the label has to bond to the viscous plastic material of the molten tube. Afterwards, the mold is opened and the projecting length is cut off at the molded opening. The molded and labeled container is ejected and cools.
In this blow molding process, the pressure during inflation of the molten tube is approx. 4-15 bar and the temperatures are significantly lower than in injection molding. The plastic materials have a lower MFI than in injection molding to form a dimensionally stable molten tube and therefore behave differently in the cooling process than the low-viscosity materials for injection molding.
In deep drawing, unoriented thick plastic sheets, mostly cast PP or PS (polystyrene), of a thickness of approx. 200-750 μm are heated and by means of vacuum or molding plug tools drawn or pressed into an appropriate forming tool. Here as well, the individual label is inserted into the mold and bonds to the actual container during the molding process. Significantly lower temperatures are used, so that adhesion of the label to the container can be a critical factor. Good adhesion has to be ensured even at these low processing temperatures. The processing speeds of this process are lower than with injection molding.
In principle, films made of thermoplastic materials can also be used in deep drawing to label containers during forming. For this, the films must have a selected property profile to ensure that label film and deep-drawn molded article fit snugly to one another in smooth and bubble-free fashion during deep drawing and bond to one another.
Frequently, adhesion of the label to the container is inadequate since in deep drawing, temperatures and pressures which are lower compared to those in injection molding or blow molding processes are used. Furthermore, similar to blow molding, air inclusions between label and container occur, which affect both visual appearance of the labeled container and adhesion. Labels for deep drawing applications are therefore provided with special adhesive layers, which ensure good adhesion to the container.
Such a film is described for example in WO 02/45956. The cover layer of this film has improved adhesive properties compared to various materials. The cover layer comprises as main component a copolymer or terpolymer of an olefin and unsaturated carboxylic acids or esters thereof. It is described that due to the improved adhesion, this film can also be used as label in deep drawing.
WO 2006/040057 describes the use of a biaxially oriented film with a microporous layer as label in deep drawing. The microporous layer comprises a propylene polymer and at least one β-nucleation agent. The microporosity is produced by transforming β-crystalline polypropylene when the film is stretched. The porous structure of the film avoids air inclusions. The adhesion of the film is surprisingly good. The porous film can therefore be advantageously used in deep drawing.
WO 98/32598 describes an in-mold label comprising at least two layers, a sealable cover layer and a base layer. The sealing layer is applied to the surface of the base layer and comprises a polyolefin having a melting temperature of less than 110° C. 25% or less of this polyolefin should melt at a temperature of less than 50° C. It is not explicitly mentioned that this film can also be used in deep drawing processes. The cover layer can be selected from a large variety of different polymers.
EP 0 889 831 describes an in-mold label comprising at least two layers. A first layer is a heat-sealing layer, which is supposed to be activatable at a temperature of 57 to 100° C. The second layer comprises vacuoles and a non-spherical vacuole-initiating particle. The film is supposed to have a thermal conductivity of less than 1.25*10-4 kcal/sec cm*° C. As an example for in-mold processes, only blow molding is mentioned. Deep drawing applications do not explicitly follow from the text. The text does not explain what is meant by activation temperature.
U.S. Pat. No. 6,764,760 describes in-mold labels of multi-layered films comprising a base layer, an intermediate layer and a sealable cover layer. As examples for in-mold processes, blow molding, injection molding and deep drawing are mentioned. The sealable layer should be embossed to avoid bubbles. The sealing layer is composed of polyethylene having a melting point of 50-130° C. Optionally, the sealable cover layer can additionally comprise modified polymers such as for example EVA or acrylate-modified polymers.
In prior art, various films are described that in general can be used as in-mold labels. In the disclosed teachings, it is frequently not differentiated between the different forming processes, and it is hence taught that the in-mold labels can be interchangeably and equivalently used in the different processes. Within the scope of the present invention it was found that this is definitely not the case. For example, there are films that work superbly in injection molding but cannot be used in blow molding because of too many bubbles and poor adhesion. Films that work in blow molding exhibit in deep drawing processes an adhesion that is too low. In principle, each forming process requires a special film, the properties of which are optimized for the exact conditions in the respective application.
Those skilled in the art generally proceed on the assumption that the applications become increasingly more critical with respect to bubble formation and adhesion in the order of injection molding, blow molding, deep drawing since the conditions during forming of the container become more “moderate” in this order. In deep drawing, the lowest temperatures and the lowest pressures are used, so that the requirements with regard to label adhesion and bubble-freeness are no longer met by the common in-mold labels in deep drawing applications. To date, in practice in-mold labeling has therefore not prevailed in deep drawing. In practice, deep-drawn containers are either directly printed upon or provided with a wrap-around label.
The few known solutions that meet the requirements technically include complex measures for the production of the film, and hence are simply too expensive to substitute direct printing or wrap-around labels. The acrylate-modified polymers according to WO 02/45956 have such strong adhesion and tack that the film has too strong a tendency to stick to the rollers during production and becomes completely unusable because of blocking on the roll that is too severe. In addition, the film, if processing into a label stack is successful, is not easily unstacked. Attempts to reduce the adhesive force using blends or additives to such an extent that the film can be handled and unstacked during production, processing and application affect the adhesive force in such a way that the film is no longer suitable for deep drawing applications since the adhesion to the container is too low.
The porous film according to WO 2006/040057 can only be produced with extremely slow production speeds since the β-crystallites are only formed in sufficient quantity by slow cooling of the extruded melt. The film according to U.S. Pat. No. 6,764,760 works for the deep drawing application only with the proposed embossing, which as an additional processing step also makes the film more expensive.
The known solutions, which were specifically developed for deep drawing, consequently rely on extreme tack of the film surface or on extreme surface roughness, or on special surface structures. Furthermore, there is a need to provide a film that is suitable for in-mold labeling in deep drawing and meets the requirements with regard to adhesion, unstackability and bubble-freeness and can be produced by the customary cost-effective film production processes.